High contrast direct positive by using active cations in silver halide solvent



United States Patent 3,418,124 HIGH CONTRAST DIRECT POSITIXE BY USING ACTIVE CATIONS IN SILVER HALIDE SOLVENT Heman Dowd Hunt, Eatontown, N.J., assignor to E. I.

du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Filed Aug. 11, 1964, Ser. No. 388,919

6 Claims. (Cl. 96-64) ABSTRACT OF THE DISCLOSURE Process for forming positive images comprising the steps of (a) exposing, imagewise, a photosolubilizable layer containing light-sensitive silver halide crystals having associated therewith a silver salt of an organic mercaptan compound and (b) prior to any reducing step treating the layer with an aqueous solution of sodium thiosulfate and an inorganic cation, e.g., potassium, cesium or rubidium.

This invention relates to improved processes involving the formation of silver halide images and to compositions for dissolving exposed silver salt images.

The principal processes of photography are based on the use of colloid-silver halide emulsion layers. In the prior art processes, a latent image is formed by imagewise exposure of a radiation-sensitive silver halide emulsion layer. Silver halide layers bearing a latent image are developed to silver by selective reduction in these instances. In the prior processes of photography, the unreduced silver remaining after development of the latent image is removed by a silver halide solvent or rendered insensitive or transparent by treatment with complexing agents. Optional aftertreatments include intensification, reduction, toning and tinting. However, the primary or first step in image formation always has been based on the selective reduction step.

An entirely different type of photographic process has been described in assignees copending application of Blake, U.S. Ser. No. 236,420, filed Nov. 8. 1962, U.S. Patent 3,155,507, Nov. 3, 1964. The novel process of said application, which embodies photosolubilization, requires the use of a specially prepared silver halide photosolubilizable emulsion layer containing a determined amount of an organic compound which reduces the dissolution rate of the silver halide grains to the extent that the silver halide does not dissolve appreciably under normal fixing conditions. Such an element is given an imagewise exposure and the silver halide in the exposed areas is then removed by dissolution and extraction under normal fixing conditions so as to yield a positive, silver halide image (the silver halide remaining undissolved in the unexposed areas). By means of additional processing steps, the silver halide image may be intensified, e.g., by reduction to convert it to a black, metallic silver image.

It is an object of the present invention to provide a new process for the production of a photographic image. Another object is to provide novel processes for producing photographic images that do not depend on conventional selective reduction of exposed silver salt images. More specifically, it is an object to provide improved solubilizing compositions and means of processing photosoluble elements to provide positive images which require less exposure, have increased contrast, increased apparent speed and decreased minimum density. Still further objects will be apparent from the following description of the invention.

A positive image is produced according to the process of this invention comprising, in the order stated, the sequential steps of:

(a) Exposing, imagewise, to actinic radiation, a photosolubilizable layer containing silver halide crystals containing at least 0.1% by weight of silver bromide having associated therewith so as to dissolve more slowly in a silver halide solvent a silver salt of an organic mercapto compound, said salt being of lower solubility in water than AgCl; the silver halide crystals so associated With the silver salt dissolving more slowly in 10% aqueous sodium thiosulfate than untreated silver halide crystals at a predetermined pH and the associated silver salt being present in such an amount, in terms of the ratio of its weight to the surface area of said silver halide crystals, that when admixed in such ratio with an aqueous silver chlorobromide (/30 mole percent) gelatin emulsion containing 57 g. of gelatin per mole Ag and .57 mg. of Ag per ml., and said silver chlorobromide dispersion is treated with 10%, by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver and mg. of sodium thiosulfate), at least three times the amount of silver chlorobromide remains undissolved as compared with a similar dispersion successively treated with 5%, by weight, aqueous sodium hypochlorite and 10%, by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver, 25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate), after vigorous and essentially equal agitation of both dispersions for 30 seconds at 25 C.;

(-b) Prior to any reducing step, treating said element with an aqueous solution comprising sodium thiosulfate and an inorganic cation selected from the class consisting of potassium, cesium, rubidium, thallium (I), strontium, and lead (II), thereby efiecting solubilization of silver halide in the more exposed areas at a rate substantially greater than in the less exposed areas until a positive image comprised of silver halide is produced.

The concentration of the inorganic cation used in combination with the silver halide solvent may vary over a considerable range, e.g., from 0.001 to 2.0 normal. The particularly preferred cation, potassium, is used most efiiciently in a range of 0.1 N to 0.4 N. Suitable salts of the above cations are those which are soluble in the thiosulfate solution and do not react with thiosulfate, e.g., the acetates and nitrates. The concentration of sodium thiosulfate may vary widely, e.g., from 0.02 to 5.0 moles per liter.

Imagewise solution of the exposed silver halide/organic compound stratum may be effected by the use of one of the above inorganic cations in combination with sodium thiosulfate and of the adjuvants commonly used in photographic fixing solutions.

For example, the aqueous solvent solution may contain in addition to the silver halide solvent, sodium sul fite, a gelatin hardener, e.g., alum, a bufiering agent, e.g., boric acid, a wetting agent, e.g., saponin or an alkyl aryl polyethoxy sulfonate, an acid, e.g., acetic acid, and other adjuvants.

Photosolubilizable compositions, emulsions, layers, and coated elements suitable for the process of the present invention have been disclosed in the following copending applications of the assignee: Blake, U.S. Ser. No. 236,412 filed Nov. 8, 1962, U.S. Patent 3,155,514, Nov. 3, 1964, Blake, U.S. Ser. No. 236,418 filed Nov. 8, 1962, U.S. Patent 3,155,516, Nov. 3, 1964, Blake, U.S. Ser. No. 317,817 filed Oct. 21, 1963, U.S. Patent 3,155,518, Nov. 3, 1964, Blake U.S. Ser. No. 317,824 filed Oct. 21, 1963, U.S. Patent 3,155,519, Nov. 3, 1964, Celeste and Cohen, U.S. Ser. No. 236,417 filed Nov. 8, 1962, U.S. Patent 3,155,515, Nov. 3, 1964.

In these photosolubilizable elements, the organic mercaptan is in association with the silver bromide crystal so that in unexposed areas, solution rate is markedly slower than normal under conventional fixing solutions. Said organic mercaptan is characterized as being present in such an amount, in terms of the ratio of its Weight to the surface area of the silver halide crystals, that when admixed in such ratio with an aqueous silver chlor-o'bro m'ide dispersion, 70/30 mole percent, gelatin emulsion containing 0.29 mg. of Ag in one-half ml., and said silver chlorobromide dispersion is treated with 10% by weight aqueous sodium thi-osulfate (so that the resulting mixture contains 0.29 mg. of Ag and 100 mg. of sodium thiosulfate), at least 3 times the amount of silver chlorobromide remains undissolved as compared with a similar dispersion successively treated with 5% by weight aqueous sodium hypochlorite and by weight aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of Ag, mg. sodium hypochlorite and 100 mg. of sodium thiosulfate) after vigorous agitation of both dispersions for seconds at 25 C. In the preferred elements any heavy metal salts present before imagewise exposure are noble metal salts.

The process of the present invention is primarily concerned with the formation of a positive silver halide image. Such an image may be viewed directly, e.g., by projection (if on a transparent support). In preferred embodiments, however, the image may be intensified by various means. A preferred method of intensification involves the conversion of silver halide to metallic silver by treatment in a fogging developer, e.g., a high pH, l-phenyl 4 methyl 3 pyrazolidone/hydroquinone developer containing iodide ion or by fogging the emulsion by exposure to light and then treating with a silver halide reducing agent, e.g., a conventional silver halide developer.

The silver halide image may also be intensified by toning, e.g., with sodium sulfide, sodium selenide, etc. In addition, color images may be obtained by developing the treated residual silver halide with a primary aromatic amine color developing agent in the presence of a color coupling compound either in the developing bath or previously incorporated in the emulsion.

Both in the primary process of forming the silver halide image and in supplementary steps of intensification, it may be desirable to insert one or more washing steps. For example, it may be beneficial to wash the element after treatment with the sodium thiosulfate solution which forms the silver halide positive image. Another washing step may be employed following intensification.

Intensification of the silver halide image may be carried out by using any chemical reducing agent capable of reducing silver ion to silver metal, e.g., hydroquinone, n-methyl-p-aminophenol, sodium hydrosulfite and stannous chloride. The function of the reducing agent may be enhanced by modifying the surface properties of the treated, residual silver halide crystals by means of alcohol, thiourea, potassium iodide, etc. Although photographic developing solutions have been found to be the preferred means of effecting the reduction, it should be noted that this is a non-selective reduction process wherein all of the remaining silver halide is converted to metallic silver. This is considerably different from the conventional photographic process wherein silver halide is differentially (imagewise) converted to metallic silver by a selective reduction. Many of the difficulties involved in this selective reduction process are eliminated in a process involving total reduction.

The novel silver halide layers useful in the processes hereof are made from emulsions of specifically treated light-sensitive silver halide crystals in a water-permeable organic colloid and they are prepared by admixing with the silver halide an organic mercaptan of the foregoing kind. The compound can be added to the aqueous colloid silver halide emulsion prior to coating onto a suitable support or by bathing or impregnating the emulsion layer with a solution, e.g., an aqueous alcoholic solution of the compound. The amount of organic mercaptan in the silver halide emulsion may vary considerably, depending upon the particular organic compound, the size and nature of the silver halide crystals, the presence of other materials which may partially cover the surface of the silver halide crystal, and upon various other factors.

The present invention embodies a broad new photographic process and is not limited to a narrow class of organic mercapto compounds with which the silver halide crystals may be treated in carrying out the invention. Instead, a large number of compounds can be used and their utility can be readily determined by a relatively simple test. Essentially, the test consists of two separate steps, which will be designated Test A and Test B. In Test A, the candidate organic compound must render a dispersion of silver halide crystals insoluble in a silver halide solvent, e.g., an aqueous solution of sodium thiosulfate, at some pH between 1 and 13. If the candidate compound meets the insolubility requirements of Test A, it must also meet the requirements of Test B by forming with said dispersion of silver halide crystals a reaction product which, upon treatment with an aqueous solution of sodium hypochlorite, becomes soluble when subsequently treated with aqueous sodium thiosulfate. The following practical tests are provided in further exemplification of the invention and include specific concentrations of solutions, times, etc., so that the utility of organic compounds may be readily and positively identified.

Test A A solution nearly saturated at 25 C. with a candidate organic compound is prepared using ethanol, acetone, dimethyl formamide, water or other suitable solvents. Depending on the solubility, a solution concentration from 0.01 to 10 percent by weight is obtained. Twenty five ml. of a gelatin silver chlorobromide dispersion containing 25 mg. of silver halide (calculated as silver bromide), prepared as described below, is treated with small increments (i.e., about 0.1 to 0.2 ml. at a time) of the said candidate solution under safelight conditions (Wrat ten l-A filter or equivalent) until the silver halide dispersion either is rendered insoluble in 10% aqueous sodium thiosulfate or the candidate is found not to cause insolubilization. Generally insolubilization will occur upon the addition of 0.05 g. or less of said candidate compound, calculated as the pure compound. Compounds which must be used in substantially greater quantities than this, e.g., l-2 g. to effect insolubilization are considered less preferred compounds. The silver halide dispersion insolubility is determined by taking a 0.5 ml. portion of the silver halide dispersion (after each incremental addition of the candidate organic compound), adding about 0.1 to 0.2 ml. of 10% aqueous sodium thiosulfate solution and observing the turbidity after 30 seconds.

As a control, there is used 25 ml. of water to which small increments of the candidate solution are added. Half-milliliter portions of the control are treated in the same manner with the sodium thiosulfate solution. The presence of visual turbidity relative to the control solution is sufiicient to satisfy the definition of insolubility in this test.

This test may be repeated for various pH increments from 1 to 13. Although there is some optimum pH value at which the test is most sensitive, this is not a sharp maximum which must be precisely attained. Rather, it has been found that there is a fairly broad range of pH values (e.g., 2.0 to 3.0 pH units) over which the test has a satisfactory sensitivity. In practice, the silver halide dispersion might be tested without adjustment (e.g., at pH 5.0 to 7.0) and if insolubilization occurs here, Test A is completed. If there is no insolubilization, the test is repeated at a higher pH (e.g., about pH 11-13). If there is still no insolubilization, the test is conducted with emulsion adjusted to a lower pH (e.g., about pH l-3). Thus three different pH values represents a practical maximum number which must be investigated to determine whether or not insolubilization will occur.

Test B An organic compound capable of insolubilizing a silver halide dispersion according to Test A is now ready for the next test, which again will be conducted under safe light conditions. To the above silver halide dispersion, there is added the minimum amount of a solution of the candidate organic compound found necessary for insolubilization. Half-milliliter samples of the dispersion are placed in two test tubes. To one sample is added 0.5 ml. of water; to the other is added 0.5 ml. of a 5% by weight aqueous solution of sodium hypochlorite. Next, there is added to both samples, 1.0 ml. of an aqueous by weight solution of sodium thiosulfate. If, after standing for up to 30 seconds, the sample treated with sodium hypochlorite clarifies (or becomes less turbid) relative to the control sample, the candidate organic compound meets the requirements of Test B and is satisfactory for use in accordance with the invention.

The silver chlorobromide dispersion referred to in the above test is a lithographic emulsion having a silver halide composition of 30 mole percent AgBr and 70 mole percent AgCl and having 20 grams of gelatin present per mole of silver halide for the steps of precipitation and ripening. This emulsion was freed of unwanted, soluble, by-product salt by a coagulation and wash procedure as taught in Waller et al., US. Patent 2,489,341, wherein the silver halide and most of the gelatin was coagulated by an anionic wetting agent, sodium lauryl sulfate, using an acid coagulation environment. Following the washing step, the emulsion coagulant was redispersed in water together with 47 grams of additional bulking gelatin. For use in the above test, this dispersion was diluted to the extent that 1 milliliter of dispersion contained 1 mg. of silver halide (calculated as silver bromide, or 0.58 mg. of silver).

Organic compounds useful in this invention, by being capable of passing both the insolubilization and the solubilization tests above (Test A and Test B, respectively), may be found among a wide variety of mercapto compounds.

The most suitable organic compounds have at least one form which can be represented by the general formula in which formula the monovalent radical R is an organic radical having a carbon atom linked directly to S. Tautomeric forms of the compound may result in different linkages for the hydrogen atom than shown in the formula. The scope of this invention, however, comprehends any organic compound which successfully passes screening Tests A and B.

Particularly preferred organic compounds of the formula are those which form silver salts of the formula R-SAg Among useful silver salts of the above formula are those which are insoluble in aqueous ammonium hydroxide at pH 12.

Dispersed crystals of silver halide, treated with an appropriate amount of suitable organic compound are affected by exposure of a portion of said crystals to actinic radiation, e.g., ultraviolet, visible, infrared, X-radiation, etc., to such an extent that at least 20% of the less soluble crystals remain when 90% of the more soluble crystals dissolve when treated in 10% by weight aqueous sodium thiosulfate solution. Useful organic compounds include mercaptans and their tautomers and especially the mercaptothiazoles, mercaptotriazoles and mercaptotetrazoles. Suitable specific compounds from these classes include 6 those disclosed in Chilton et al., US. Patent 2,432,506. Other useful compounds include mercaptooxazoles, naphthalene thiols, phenanthrene thiols, anthracene thiols and thioureas.

The chemical testing by Test A and B above for selecting suitable compounds has been found to give absolute correlation, i.e., organic compounds which have been subjected to the pair of tests have produced without exception, when tested in actual photographic emulsions, the very effects predictable by said tests. Combinations of the compounds with various basic dyes, e.g., dyes of the cyanine class and Methylene Blue (Colour Index Basic Blue 9), Crystal Violet A0 (Colour Index Basic Violet 3) and Rhodamine 6 GDN Extra (Colour Index Basic Red 1) have proven useful.

The organic compound is associated with the silver halide crystals in greater than fog inhibiting amounts, the latter amounts being the maximum quantity which provides low fog without serious loss in speed.

The invention will be further illustrated but is not intended to he limited to the following examples.

EXAMPLE I Forty grams of photographic gelatin were dissolved in 900 grams of water. To this aqueous gelatin solution there were added 168 g. of a precipitated and washed silver halide coagulate containing 0.48 mole of silver halide, with a 7:3 AgCl2AgBr mole ratio. After stirring for 10 min. at 110 F., there was admixed 22 ml. of a solution composed of 1 g. Z-mercapto-4-phenylthiazole dissolved in 100 ml. ethanol. The emulsion was digested for 20 minutes at 160 F., then adjusted with water to a total weight of 1220 g.

To a 305 g. portion of this emulsion there was added 20 ml. of a 1% by weight chrome alum solution, and the emulsion was coated at F. on a vinylidene chloride copolymer subbed polyethylene terephthalate film made according to Example IV of Alles, US. Patent 2,779,684. The silver halide coating weight was mg./dm. calculated as silver bromide.

Specimens were exposed through a standard 21-step square-root-of-two step tablet to 2800 K. light at an intensity of 16,146 meter-candles for 10 seconds. Three such exposures were subjected to the following steps at 68 F., identical except for the composition of the silver halide solvent.

Processing step: Duration Silver halide solvent minutes 1 Water wash do 1 Re-exposure, 500-watt incandescent lamp (#2 photoflood) at 24" seconds 10 Intensifying solution minutes 2 Water wash do 5 The composition of the intensifying solution is:

H O ml 800 Na SO anhyd. g 80 Hydroquinone g 16 4-methyl-1-phenyl-3-pyrazolone g 1 Boric acid g 5.5

KBr g 2 KI g 2.5

6-nitrobenzimidazole nitrate solution (containing 1 g. solute dissolved in 100 ml.

of ethanol) ml 40 1-phenyl-5-mercaptotetrazole solution (containing 1 g. solute dissolved in 100 ml.

of ethanol) -5 ml 10 NaOH g 24 H O to make 1 liter.

Transmission densities of the product image steps were measured using a densitometer (Western Electric RA- l100-C) with a. visual yellow filter.

OPTICAL DENSITIES A photosensitive gelatino-silver halide coating was prepared similar to that of Example I except that the 2- mercapto-4-phenylthiazole concentration was reduced from 460 mg./mole silver halide to 417 mg./mole, and the emulsion was ortho sensitized by addition of a thiazoline-rhodanine merocyanine dye in the concentration of 25 mg./mole silver halide. Exposure and processing were identical to Example I except for the composition of the silver halide solvent. Density readings were similarly obtained.

OPTICAL DENSITIES Composition of 0.25M NaQSZO; 0.25M Na2S2O;, 0.25M NazszOg,

aqueous silver (control) 0.002M TlNO; 0.02M Pb(NO )2 halide solvents Ste 0. 10 0. O7 0. l 4. 00+ 3. 35 2. 0G 4. 00+ 2. 95 1. 28 3. 00 2. 30 0. 59 1. 92 1. 50 0. 26 1. 41 0. 69 0. 12 0. 90 0. 22 O. 07 O. 61 0. 09 O. 06 0. 33 O. 07 0. 06 0. 20 0. 10 0. 08

EXAMPLE III of the silver halide solvent.

Processing step: Duration Silver halide solvent "seconds" 45 Wash do 30 Re-exposure, 500-watt incandescent lamp do 1 Intensifying solution minutes 2 Wash (water) do The intensifying solution is similar to that of Example I but contains no KI.

The silver halide solvent solution employed as the control has the composition:

H O ml 300 Na S O anhyd. g 77 Na SO anhyd. g 7.5 NaAl(SO -12H O g Na2B407 g 9 Glacial acetic acid g 6 H O to make 1 liter.

Transmission densities of the product image steps were measured using a densitometer (Eastman Kodak Model B).

The other exposed film specimens were treated in similar solutions of silver halide solvent but which contained,

additionally, either cesium or rubidium salts in the molarity indicated below.

In the re-exposure step referred to above, the lamp was disposed six inches from the surfaces of the film specimens.

OPTICAL DENSITIES Composition of Control Control aqueous silver Control 0.05M CsNOs 0.08M RbCl halide solvent Step:

Base-Hog 0. 13 0.13 o. 13

EXAMPLE IV A gelatino-silver bromo-chlonde photographic emulsion similar to that of Example I (with a 7:3 AgCl2AgBr mole ratio) contained one mole of silver halide and 80 g. of gelatin. This emulsion was digested for 20 minutes at 160 F. in the presence of 24 mg. of a thiazoline-rhodanine merocyanine dye and 0.54 g. of the insolubilizing mercaptan, 4-(p-bromophenyl)-2-mercaptothiazole. Upon the addition of the usual post-digestion adjuvants, the emulsion was coated at 60 mg./dm. (calculated as AgBr) on the film base described in Example I and labeled as Coating A.

Other coatings were prepared in a similar manner but with the use of a different insolubilizing mercaptan in the concentrations as listed below:

Grams of mercaptan Coating Insolubilizing mercaptan per mole of silver halide A 4(p-bron1ophenyl)-2-mercaptothiazole 0. 54 4-eyclohexyl-2-mercaptothiazole 0. 47 4-isobutyl-2mercaptothiazole O. 54 4-isopropyl-2mercaptothiazole. 0. 47 E Z-mercaptO-u-naphthothiazole 0.55

CSH N F bcnzyl dithiocarbazate 0. 51

i CHzS-CNHNH) G 2-mercapto-4,5-diphenylimidazole 0. 66

H 2-mercapto-4-phenyloxazole 0. 46

Also, coatings E, F, G and H differed from Coating A in that they contained none of the merocyanine sensitizing dye and were applied to the film base at a coating weight of about 77 mg./dm. (calculated as silver bromide).

Two samples of each coating, A through H, were processed identically except for the treatment in the silver halide solvent. One strip from each coating was processed in a silver halide solvent like that employed as the control in Example III except that it additionally contained 10 g. per liter of sodium acetate. The other strip from each coating was processed in a solution like the control of Example III except that it additionally contained 10 g. per liter of potassium acetate. Optimum processing times in the silver halide solvent varying from 25 to seconds were employed for the various coatings but the members of each pair of samples from a given coating were processed for exactly the same length of time.

The total processing sequence was as follows:

Step 1.Expose for 10 seconds through a standard 21-step square-root-of-two step tablet using a SOC-watt (#2 Photoflood) lamp at 15 inches distance.

Step 2.Form silver halide images by treating for optimum time in the silver halide solvent solutions described above, one strip from each coating in the solution containing sodium acetate and the other strip from each coating in the solution containing potassium acetate.

Step 3.-Wash for 30 seconds in water.

Step 4.Uniformly expose for 3 seconds (flash exposure) using a SOO-watt incandescent lamp (#2 Photoflood lamp) at 15 inches.

Step 5. Intensify silver halide images by treatment for one minute at 68 F. in a photographic developer solution like that described in .Example 11 except that it does not contain KI.

Step 6.Wash for 5 minutes in distilled water.

Step 7 .Dry in air at room temperature.

Optical transmission densities of the images were obtained as in Example I.

For each of these processed samples of Coatings A through H, a pair of sensitometric curves (Hurter and Driffield type with optical transmission density vs. log of exposure) were plotted so that comparisons could be made of samples processed in the two silver halide solvent solutions. For all eight coatings it was evident that the solvent solution containing potassium acetate gave better clean-out than the solvent solution containing sodium acetate. At all exposure levels the densities were lower with the potassium acetate than with the sodium acetate solution but the superiority of clean-out was evidenced by the fact that a smaller difierence in density in each pair of curves occurred in areas receiving minimum exposure than in areas receiving greater exposure.

The silver halide should, preferably, contain silver bromide, but it may be either a pure silver bromide or a mixed halide system conventional in photographic practice containing silver bromide. Silver chlorobromide is particularly preferred. While, for rapid processing, a high silver halide to binder ratio is preferred, more conventional ratios can also be used.

In place of part of the gelatin, other natural or synthetic water-permeable organic colloid binding agents can be used and in some cases such binders can be used alone. Such agents include water-permeable or water-soluble polyvinyl alcohol and its derivatives, e.g., partially hydrolysed polyvinyl acetates, ethers and acetals having a large number of intralinearCH CHOHgroups, hydrolysed, interpolymers of vinyl acetate and unsaturated addition polymerizable compounds, e.g., maleic anhydride, acrylic and methacrylic acid esters and styrene. Suitable such colloids of the last-mentioned type were disclosed in US. Patents 2,276,322; 2,276,323 and 2,397,866. The useful polyvinyl acetals include polyvinyl acetaldehyde acetal, polyvinyl butyraldehyde acetal and polyvinyl sodium o-sulfobenzaldehyde acetal. Other useful colloid binding agents which can be used include the poly-N-vinyllactams of Bolton US. Patent 2,495,918, various polysaccharides, e.g., dextran, dextrin, etc., the hydrophilic copolymers in Shacklett US. Patent 2,833,650, hydrophilic cellulose ethers and esters, and polymers of acrylic and methacrylic esters and amides. It has been found practical to treat silver halide layers on a base material in the essential absence of a binder, e.g., by chemical or vacuum deposition.

The emulsions may optionally contain any of the usual adjuvants customarily employed in silver halide systems so long as they do not interfere with the adsorption and complexing action of the organic compound which modifies the silver halide solubility.

The emulsions can be coated on any suitable support, e.g., cellulose esters, cellulose mixed esters; superpolymers, e.g., poly(vinyl chloride co vinyl acetate), polyvinyl acetals, butyrals; polystyrene; polyamides, e.g., polyhexamethylene adipamide; polyesters, e.g., polycarbonates, polyethylene terephthalate, polyethylene terephthalate/ isophthalate, esters formed by condensing terephthalic acid and its derivatives, e.g., dimethyl terephthalate with propylene glycol, diethylene glycol, tetramethylene glycol, cyclohexane-1,4-dimethanol (hexahydro-p-xylene dialcohol); paper, metal, glass, etc.

Elements suitable for this novel process can be prepared by bathing a photographic film in a solution of an appropriate mercaptan compound. In this embodiment, the silver halide crystals near the surface of the coated emulsion stratum are in contact with a higher concentration of the mercaptan compound. Crystals farther from the surface are treated with less of the mercaptan compound and, if the rate of diffusion is sufficiently slow, there may be considerably less of the mercaptan compound (even approaching zero) reacting with the lower than with the surface silver halide crystals. In such elements, satisfactory results might be obtained with only a fraction, e.g., one-half, of the amount of the mercaptan compound theoretically calculated as required to just cover the surface of a mole of the silver halide crystals.

The photographic processes of the invention have advantages over previously known systems based on selective reduction of exposed silver halide for forming either direct positive or negative images without resorting to the special effects and sensitizing procedures previously used for preparing such images. Since direct-positive image formation does not require selective reduction, this present process is not limited to the use of certain photographic developing agents but may be accomplished by using a wide range of reducing agents. Many such compounds are of very low cost and can be used to form images of much higher covering power than customary, thus efiecting important economies in processing, as well as greatly increasing the efiiciency of the silver image with a resultant increase in sensitivity.

It is obvious from the examples above that the combination of a cation selected from the disclosed group, in combination with the silver halide solvent solution, effects a considerable improvement in the processing of exposed, photosoluble elements. Photosolubilization is effected by the imagewise destruction of an insolubilizing barrier on the surface of silver halide grains. The difference in rate of solution between exposed and unexposed grains produces an image. This difference in rate is increased when, according to the present invention, any of the class of positive ions disclosed herein are present in the solution of silver halide solvent. Increasing this rate of solution brings about an improvement in contrast, apparent photographic speed and gives better (lower) minimum optical densities in areas of heavy exposure.

Another advantage of this invention is that it provides a process for forming images that does not require special equipment but instead can be used with conventional equipment and apparatus. A further advantage is that the processes can be carried out successfully by photographic technicians and photographers of ordinary skill with, in many instances, the exposing and processing steps beings carried out under ordinary room light conditions, A still further advantage is that the processes utilize conventional reducing agents, e.g., developers and fixing agents. Still additional advantages will be apparent from the above description of the invention.

I claim:

1. A process for the formation of positive images which comprises, in the order stated, the sequential steps of (a) exposing, imagewise, to actinic radiation, a photosolubilizable layer containing light-sensitive silver halide crystals consisting of at least 0.1 percent of silver bromide having associated therewith a silver salt of an organic mercapto compound; and

(h) prior to any reducing step, treating said element with an aqueous solution comprising sodium thiosulfate and an inorganic cation selected from the class consisting of potassium, cesium and rubidium, cations, thereby effecting solubilization of silver halide in the more exposed areas at a rate substantially greater than in the less exposed areas until a positive image comprised of silver halide is produced.

2. A process for the formation of images which comprises, in the order stated, the sequential steps of (a) exposing, imagewise, to actinic radiation, a photosolubilizable layer containing light-sensitive silver halide crystals consisting of at least 0.1 percent of silver bromide having associated therewith a silver salt of an organic mercapto compound; said salt being of lower solubility in water than silver chloride; the silver halide crystals so associated with the silver salt dissolving more slowly in 10% aqueous sodium thiosulfate than untreated silver halide crystals at a predetermined pH and the associated silver salt being present in such an amount, in terms of the ratio of its weight to the surface area of said silver halide crystals, that when admixed in such ratio with an aqueous silver chlorobromide (70/30 mole percent) gelatin emulsion containing 57 g. of gelatin per mole and .57 mg. of Ag per ml., and said silver chlorobromide dispersion is treated with 10%, by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver and 100 mg. of sodium thiosulfate), at least three times the amount of silver chlorobromide remains undissolved as compared with a similar dispersion successively treated with by Weight, aqueous sodium hypochlorite and by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver, 25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate), after vigorous and essentially equal agitation of both dispersions for 30 seconds at 25 C., and

(b) prior to any reducing step, treating said element with an aqueous solution comprising sodium thiosulfate and an inorganic cation selected from the class consisting of potassium, cesium and rubidium cations, thereby effecting solubilization of silver halide in the more exposed areas at a rate substantially greater than in the less exposed areas until a positive image comprised of silver halide is produced. 3. A process according to claim 1 wherein said organic compound is a mercapto compound of the formula wherein R is a hydrocarbon nucleus of 4-12 carbon atoms.

4. A process according to claim 1 wherein said organic compound is Z-mercapto-4-phenylthiazole.

5. An aqueous silver salt solvent solution consisting essentially of sodium thiosulfate in an amount of 0.02 to 5.0 moles per liter and an inorganic cation selected from the group consisting of potassium, cesium .and rubidium cations in an amount to form a 0.001 to 2.0 normal solution.

6. An aqueous silver salt solvent solution consisting essentially of sodium thiosulfate in an amount of 0.02 to 5.0 moles per liter and potassium acetate in an amount to form a 0.001 to 2.0 normal solution.

References Cited UNITED STATES PATENTS 3,155,518 11/1964 Blake 96107 FOREIGN PATENTS 477,628 1/ 1938 Great Britain.

OTHER REFERENCES Gauvin, H., Activity of Developers in Relation to Their Metallic Ions and Their pH, in Science et Inds. Phot. 22, 41-6 1951 Abribat et al., Acceleration of Fixing Process by Active Cations, in Royal Phot. Soc., London, 1955, pp, 177-80.

NORMAN G. TORCHIN, Primary Examiner.

R. E. FICHTER, Assistant Examiner.

U.S. Cl. X.R. 9661 

